The present invention relates to a fingerprint sensing device comprising an array of sense elements which each comprise a sense electrode spaced from a sensing surface over which a finger whose print is to be sensed is placed and providing in combination with an overlying fingerprint portion a capacitance, and a transistor connected between the sense electrode and first and second address conductors via which respectively the sense element is selected by means of a selection signal and an output dependent on the capacitance of the sense element is obtained. The invention relates also to a fingerprint recognition system incorporating such a device.
A fingerprint sensing device of the above kind is described in U.S. Pat. No. 5,325,442. In this device, the sense elements are arranged in a row and column array and the transistors of the sense elements, in the form of thin film transistors (TFTs), are connected via sets of row and column address conductors to a drive circuit. The gates of the TFTs of the sense elements in one row are connected to a respective, common, row conductor while the sources of the TFTs of all sense elements in one column are connected to a respective, common, column address conductor. The drain electrode of each TFT is connected to the sense electrode of the sense element. The sense electrodes together with overlying dielectric material and individual fingerprint portions constitute capacitors. The row address conductors are connected to a scan circuit which applies a gating (selection) signal to each row conductor in a respective row address period to turn on the TFTs of the sense elements of each row in sequence. Simultaneously with a gating signal a predetermined potential is applied to the column address conductors to charge the capacitors. The individual capacitances of these capacitors depend on the spacing of the fingerprint portions from the sense electrodes, as determined by the presence of a ridge or a trough of the fingerprint, and are measured by sensing the charging current flowing in the column conductors during charging of the capacitors, using current or charge sensing amplifier circuits incorporated in the drive circuit. At the end of the row address period, the TFTs are turned off and a gating signal applied to the next row conductor to turn on the TFTs of the next row of sense elements. Each row of sense elements is addressed in this manner in turn and the variation in sensed capacitances produced over the array of sense elements by a fingerprint ridge pattern provides an electronic image or representation of the three dimensional form of the fingerprint surface. Before the sense elements are addressed again the charge on the sense electrodes is removed, or at least reduced, either by incorporating a resistor in each sense element connected between the sense electrode and ground, by changing the predetermined voltage applied to column conductors in successive read cycles, or by arranging the drive circuit to include an intermediate reset cycle between successive read cycles.
A different form of sensing element is described in WO97/40744 (PHB 34068) which uses two TFTs whose gates are connected respectively to successive row address conductors. The first TFT is connected such that when it is operated by means of a gating signal applied to its associated row address conductor it serves to charge up the capacitance formed by the sense electrode and overlying fingerprint portion, the amount of charge supplied differing according to whether a ridge or valley is present over the sense electrode. The second TFT is connected between the sense electrode and the second address conductor and is operated immediately after operation of the first TFT so as to transfer any charge stored on the capacitance to the second address line where it is sensed by a sense amplifier. Faster read-outs from the array are possible with this arrangement because the need to reset the capacitances of the sense elements in a separate step is removed.
However, the sensing operation relies on the need for the capacitance to be discharged into the second address conductor through the TFT and the time needed to achieve this can be a limiting factor. The operating speed of this device is still therefore less than ideal. Such discharge can typically take tens of microseconds and if adequate time is not allowed for this some charge may remain in the sensing element""s capacitance which could then affect a subsequent reading. Moreover, ac noise from a person""s finger is coupled via the capacitance and the TFT to the sense amplifier where it is integrated over this relatively lengthy period of time and this can lead to the distinction between read-outs for fingerprint ridges and valleys being diminished.
It is an object of the present invention to provide a fingerprint sensing device offering improvements in these respects.
According to one aspect of the present invention a fingerprint sensing device of the kind described in the opening paragraph is characterised in that the drain and source electrodes of the transistor are connected to the first and second address conductors and the gate electrode is coupled to the sense electrode. The operating principle of the sensing elements of the present invention is very different to that of the known devices. Rather than of relying on the capacitance being discharged into the second address conductor for sensing by the sense amplifier, the sensing of a ridge or valley of a fingerprint is accomplished instead by sampling the transistor""s on and off currents. The transistor is not turned on directly by means of a gating selection signal applied via an address conductor to its gate as in the known arrangements but by the effect of a person""s fingerprint. The transistor is either turned on or held off depending on the presence of a ridge or valley over the sense electrode. With a selection potential applied to the first address conductor, the effect of parasitic gate source and gate drain capacitances inherent in the transistor is to couple a charge on the gate. The resulting change in gate potential is dependent on the magnitude of the capacitance formed by the sense electrode and an overlying fingerprint portion. In the case of this portion being a ridge, the capacitance is comparatively large and consequently the change in gate potential is small and of insufficient magnitude to turn on the transistor. In the case of the portion being a valley, the capacitance is comparatively small and the change in gate voltage is thus larger, and of sufficient magnitude to turn on the transistor. This results in an electrical current flowing into the second address line where it is sensed. This drain-source current can be sampled very quickly, for example within one to five microseconds, compared to the time necessary to sense transferred charge in the known device. Consequently, a much faster read-out is possible from the array. Also, because only a short integration time is needed, much better noise rejection is obtained. The ridge/valley output ratio, i.e. the ratio of the outputs obtained from a sense element in the presence of an overlying ridge and valley of a fingerprint, is a function of the off/on current ratio of the transistor which can be many orders of magnitude, thus providing a high contrast ratio and a high signal to noise ratio.
The inherent gate/source and gate/drain parasitic capacitances of the transistor may be deliberately increased so as to ensure, and actively assist, the intended sense element operation. To this end, the gate of the transistor may be formed as an extended area of conductive material, such as a metal, covering the source and drain electrodes as well as the channel region and may serve to provide also the sense electrode.
It will be appreciated that reference to the source and drain electrodes of the transistors can be interchangeable.
As in the known devices, the sense elements are preferably arranged in rows and columns and connected to sets of first and second address conductors extending in the row and column directions with the transistors of the sense elements in a row being connected to a common address conductor of the first set and with the transistors of the sense elements in a column being connected to a common address conductor of the second set. In this case, a drive circuit connected to the sets of address conductors may conveniently be arranged to supply a selection signal to each of the address conductors of the first set in sequence so as to operate the sensing elements on a row by row basis.
In order to avoid the possibility of the gate of the transistor floating either high or low due to a build up of static electricity on a person""s finger which could affect the desired operation of the sense element, each sense element preferably further includes a further switching device, preferably another transistor, which is connected to the gate of the first-mentioned transistor and operable periodically to set the potential of the gate to a predetermined level, preferably virtual earth. In the case of the switching device comprising a further transistor, then preferably the drain and source electrodes of this transistor are connected between the gate of the first mentioned transistor and the address conductor of the first set to which the first-mentioned transistor is connected and its gate connected to another address conductor of the first set different to that to which the first-mentioned transistor is connected. Thus, when a selection signal is applied to that different address conductor so as to select and operate the sense elements associated with that address conductor, the selection signal serves also to turn on the further transistors of a non-selected row of sense elements so as to set the gate potential of the first mentioned transistors of the non-selected row. Alternatively, the drain and source electrodes could be connected between the gate of the first-mentioned transistor and the second address conductor. In this case, however, any charge is transferred to the second address conductor which may be less desirable. In another alternative arrangement, the further transistor in each sense element may be arranged with its source and drain electrodes connected between the other address conductor of the first set and the gate of the first-mentioned transistor and with its gate connected to the second address conductor. With this arrangement the gates of the first-mentioned transistors in a column of sense elements can be reset by means of a gating signal applied to the second address conductor and any charge present on the gates is prevented from passing to the second address conductor.
Desirably, the gate/source and gate/drain parasitic capacitance values of the further transistor are small compared with those of the first-mentioned transistor, which can be achieved through appropriate design, for example using known self-alignment techniques, so as not to have an undue effect on the operation of first-mentioned transistor.
Instead of using a further transistor in the sense element, problems due to static-electricity on a person""s finger could instead be avoided by providing grounded conductors on the sensing surface, for example in the form of a grid extending in spaces between row and columns of sensing elements in the array.
In a preferred embodiment, the transistors of the array of sense elements comprise thin film transistors (TFTs) which, together with the sets of address conductors and the sense element electrodes, are carried on an insulating support, for example of polymer material or glass. The TFTs may comprise amorphous silicon devices. Preferably though, the TFTs comprise polysilicon TFTs. For convenience, the drive circuit is preferably integrated on the same support and fabricated simultaneously with the sense element TFTs and the sets of address conductors and this is readily possible using polysilicon technology. Inexpensive and compact sensing devices are then obtained which are ideally suited to, for example, integration in smart cards and the like.
The sensing device may instead be fabricated as an integrated circuit using a semiconductor wafer.
According to another aspect of the present invention there is provided a fingerprint recognition system comprising a sensing device in accordance with the one aspect of the invention, means responsive to the output from the sense means of the device to provide characteristical data of a sensed fingerprint, and means for comparing said characteristical data with stored characteristical data for one or more fingerprints.